# killall -9 X

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That is a funny question to ask if you are a human (because you *should* know the answer). But it is not that dumb for a system. As a matter of fact, among the multiple tests the shell admits (man 1 test), there is a -t which serves specifically for that:
$ test -t 0
returns 0 (that is, success or true in shell jargon) if the standard input (file descriptor 0) is open and is associated with a terminal. So, unless things are going pretty bad, the following
$ test -t 0 && echo $?
Should always print a 0.
However, when a file ...